Process for increasing the mechanical resistance of elements manufactured in alloys of lead and antimony

ABSTRACT

THIS INVENTION IS CONCERNED WITH ALLOYS OF LEAD AND ANTIMONY, AND MORE CONCRETELY WITH A THERMAL TREATMENT THAT PERMITS THE ACCELEERATION OF THE PROCESSES OF PRECIPITATION OF ANTIMONY WHICH GIVES RISE TO THE HARDENING OF THESE ALLOYS, AND WHICH CONSEQUENTLY PERMITS THE OBTAINING IN THESE ALLOYS OF ELEMENTS OF HIGHER MECHANICAL RESISTANCE, WHICH MAKES THEM MORE SUITABLE FOR HANDLING IN SHORT PERIODS OF TIME.

P 1973 JOSE-MARIA s. AGUIRRE ET L 3,759,

PROCESS FOR INCREASING THE MECHANICAL RESISTANCE OF ELEMENTSMANUFACTURED IN ALLOYS OF LEAD AND ANTIMONY Filed Aug. 25, 1971 2Sheets-Sheet 1 x x- Prgv/OUS freafmenz 16 A. at T x 235 ai T I I l l 420do 0 40 20 TEMPERA TURE, "C

Fig, 1

JUAN JOSE REGIDOR ARRIBAS MIGUEL ABALLE CARIDE g l/F2544 ATTORNEY p 1973JOSE MARIA s. AGUIRRE E 3,759,752

PROCESS FOR lNCREASING THE MECHANICAL RESISTANCE OF ELEMENTS IMANUFACTURED IN ALLOYS OF LEAD AND ANTIMONY Filed Aug. 25, 1971 2Sheets-Sheet 2 A A"' 0- I5, 0

A A o HB x 0 10,0 Prev/00s freaimenz O Name A A 3/5 al 10 c X Ja cu.-10c 0' 24:75:11 -10 C (days) Fig. 2

INVENTORS JOSE MARIA SISTIAGA AGUIRRE MARCELINO TORRALBA DIAZ JUAN JOSEREGIDOR ARRIBAS MIGUEL ABALLE CARIDE ATTORNEY United States Patent Int.c1. C21d 1762,- C22f 1/12 US. Cl. 148-3 2 Claims ABSTRACT OF THEDISCLOSURE This invention is concerned with alloys of lead and antimony,and more concretely with a thermal treatment that permits theacceleration of the processes of precipitauon of antimony which givesrise to the hardening of these alloys, and which consequently permitsthe obtain- 1ng in these alloys of elements of higher mechanicalresistance, which makes them more suitable for handling in short periodsof time.

DESCRIPTION OF PREVIOUS TECHNIQUE The alloys of lead and antimony can beconsidered as some of the most widely used in the field of non-ferrousmetallurgy due to their use in the manufacture of products whoseconsumption reaches very important tonnages, such as sheaths fortelephone, telegraph and electric power cables, piping, certain elementsfor the chemical industry, munitions, bearings, printers metal, screensfor protection against penetrating radiation and grids for accumulators.Of all these applications, that which without doubt gives rise to thehighest consumption of lead is that mentioned last, for which purposethere are utilised at the present time alloys with contents of 4 to 6percent of antimony by weight, although some years ago this percentageused to be much higher. The importance of these alloys is made manifestby the fact that in Europe the consumption of lead for this purpose rosebetween 1963 and 1969 from 314,000 tons to 430,500 tons, and in the sameperiod for time consumption in the United States increased from 398,000tons to 506,300 tons.

Depending on the end sought, the types of alloys employed can be dividedinto two main categories: those intended for use in sheaths piping,etc., in which it is endeavoured to increase the mechanical resistanceof the lead, maintaining good ductility, use contents of antimony whichin general do not exceed 1.25 Percent by weight, although theyfrequently contain other alloying elements. This type of material can behot-shaped, especially by extrusion.

The other type of alloys is that in which it is desired to have the bestpossible mechanical resistance without the capacity of the alloys to beshaped being over important, since the pieces are obtained by pouringand do not receive any subsequent mechanical treatment, or at most aresubjected to a process of machining. This is the case with grids foraccumulators, and with the blocks for constructing protective screensagainst penetrating radiations, for example. In these cases contents ofantimony higher than 4 percent are employed.

The increase of mechanical resistance of these alloys by comparison withpure lead is not due to hardening by solid solution, since antimonyalone is soluble in lead at ambient temperature in very small quantitiesof the order of 0.3 percent. The hardening is produced because, on thematerial being cooled after manufacture, the anti- 'ice mony presentprecipitates in the form of small particles which distort the network ofthe lead and make its plastic deformation difficult. When antimonycontents higher than 3.5 percent are concerned, during the process ofsolidification considerable precipitates of antimony are formed whichconstitute a further contribution to mechanical resistance. It isimportant to note that the hardening due to precipitation takes sometime to be produced, tending towards a maximum value, the lower thetemperature the higher the value, but the higher the temperature thesooner this maximum value is produced. After the attainment of thismaximum, a new softening is produced. By way of example, an alloy of 2percent of antimony delays in reaching the maximum hardness, which isapproximately 16 HB, some 40 days, after having been made soluble andtempered, when it is kept at 20 C., while the same alloy kept at C.takes only 8 hours to reach the said maximum, but in this case thehardness attained is only 10 HB. For this reason it is necessary to usehigher contents of antimony to attain high values of hardness in shortperiods of time.

In the majority of the applications which we have mentioned as requiringa high mechanical resistance, the product is obtained by molding, areason for which recourse is also had to high contents of antimony,since the meltability of these alloys is greater the nearer they are tothe eutectoid state of 11.1 percent Sb. This problem is not highlycritical since it can be resolved by suitably varying the temperature ofthe molten metal and the mold, a reason for which the content has beenreduced to the above-mentioned 4 to 6 percent of antimony.

In some concrete application, such as the manufacture of grids foraccumulators, it seems that the presence of antimony gives rise tosurfaces that favour the adherence of the active material (paste), butit appears that the adherence is heterogeneous and is related to theheterogeneity of the distribution of the antimony. By way of example ofthe present situation concerning manufacturing processes, we shall nowrefer to the case of the manufacture of grids for accumulators. Thealloy of lead and antimony with contents that can vary considerablyaccording to the manufacturer, but which generally lie between 4 and 6percent of antimony, is kept molten at a temperature of some 400 or 450C. and is poured into a mold of suitable form which is kept at some 180or 200 C. Once the grid has solidified, it is expelled from the mold andcools rapidly, because of its shape. Under these conditions the coolingof the lead is very rapid, which gives rise to the fact that aconsiderable part of the antimony is retained in the form of anoversaturated solid solution, the result being that the grid has amechanical resistance lower than that which it will attain when theWhole of the antimony has precipitated. Since the operation of applyingpaste to the grids requires that the latter have a certain mechanicalresistance, since otherwise they would become deformed on paste beingapplied, producing the consequent paralyzation of the train of themachine, the grids must be kept for a certain time at a suitabletemperature so that the antimony will go on precipitating and increasingthe hardness. In the treatments of the alloys that are utilised inindustry, times are employed which are not sufiiciently long, but whichcannot be increased, due to economic reasons. Because of this, and sincewith a lower content of antimony lower values of hardness are attained,for the moment it is difficult to bring about a reduction of theantimony content of these alloys. Nevertheless, this would be desirable,even if only because of the fluctuations of an economic nature thataffect the prices of batteries. due to the price of antimony.

Returning to the concrete case of the grids, after they have been storedfor a certain time, as has been said, in order to produce hardeningthrough the precipitation of the antimony, they are taken to the pastingmachine which carries out the process in a continuous form. The otheroperations involved in the manufacture of accumulators are well-known,and are not related to the probelm being considered here.

Thus, concluding the example, we see that it would be of great interestto achieve the acceleration of the processes of hardening, which wouldimply a saving in time and, consequently, an economy.

If this is achieved, moreover, for lower contents of antimony, theimprovement is even greater. It is also possible to think of theimportance that an improvement of this type would have, when it is aquestion of pieces obtained by molding and which subsequently have to bemachined to give them a concrete shape. In such cases the harder thealloy the easier will the machining be, due to the fact that the alloywill be more easily adjustable to the machines used for machining.

SUMMARY OF THE INVENTION The present invention consists of a method ofaccelerating the processes of aging or precipitation of alloys of leadand antimony, by means of the retention in the network of agents bearingthe atoms of antimony, such as are the empty spaces introduced by arapid cooling, and which on becoming united to the atoms of antimony canproduce complexes which are more rapidly diffused through the network.The vacant spaces in the network are physical defects therein whichexist in great numbers at high temperatures, but when the material iscooled remain in a metastable equilibrium and consequently tend tobecome diffused and eliminated. This phenomenon, in the case of alloysof lead and antimony, takes place at high speed at temperatures equal toor greater than the ambient temperature, but nevertheless when thetemperature is lowered their mobility diminishes, and consequently theytend to become united to atoms of antimony, thus reducing theirmetastability and increasing the mobility of the atoms of antimony withwhich they are associated, which, since they are also initially in ametastable state after cooling, tend to become grouped together, formingprecipitates. Again, the lower the temperature at which they areproduced the smaller and more abundant are these precipitates, with theresult that there is simultaneously attained a finer distribution of theantimony.

Because of this, in order to produce a more rapid aging of the alloy oflead and antimony under consideration, it is sutficient to introduce thepiece that it is desired to age more rapidly after it has been cooled,into a bath at a temperature of -l C. for a time which can vary,according to the alloy, from a few hours to two days, and the longer theduration of the treatment the greater is the effect. The subsequentaging at ambient temperature will take place spectacularly more rapidly,the hardness value after the cold treatment will be significant, and thefinal precipitation will be finer than if this treatment had not takenplace. Again, and since the precipitation is finer, the maximum value ofhardness is greater than that presented by the same alloy when it hasnot undergone the aging treatment in a cool medium. A piece under theseconditions can be subjected to machining, or, in the case of anaccumulator grid, to the application of paste, more easily than anotherpiece which has not undergone this treatment.

DESCRIPTION OF THE PREFERRED VERSION In its widest aspect, thisinvention serves to bring about a rapid aging and hardening of pieces ofany alloy of lead and antimony. Its preferred version is concerned withthe manufacture of grids for accumulators. The modification of the pr es normal y utilised consists in Cooling the grid as rapidly as possible,both when it has been obtained by pouring and when it has been obtainedby hot-shaping. Likewise, if the piece has been obtained bycold-shaping, the process can be utilised by previously giving the piecea solution treatment and rapid cooling. In any one of the cases cited,Once the piece has been cooled it should immediaely be introduced into abath or a chamber or any other place which is at a temperature below 20C., preferably -10 C. The time that it should remain at this temperaturewill be established according to the alloy and the degree of hardness itis desired to obtain. Once this time has transpired, the grids are in acondition for paste to be applied to them. In some cases a decision maybe taken to maintain the alloy at a low temperature for a shorterperiod, followed by another period at the amibent temperature prior tothe application of the paste.

EXAMPLE I A series of wires of some 15 cms. long and 2 mm. in diameterof an alloy Pb-2 percent Sb was taken, and was submitted to a solutiontreatment at 240 C. for 1 hour. The wires were quenched in water at 10C. and immediately afterwards were introduced into a bath of methanol atthe temperatures and for the periods indicated in the graph.Subsequently, a study was made of the variation in electrical resistanceof these wires, this variation being an index of their state of aging,at the end of four days during which they were kept at ambienttemperature. It was seen that the test pieces that had been submitted toa treatment lying beween 0 C. and -10 C. underwent a much more rapidaging than did the others.

Description of the graph of Example I On the graph of FIG. 1 there isrepresented the value reached by the electrical resistance of testpieces sub jected to solution treatments and quenching which have beenimmediately subjected to the treatments whose times and temperatures arestated on the graph. On the ordinates there is represented theelectrical resistance referred to the value it has immediately after thetempering treatment. The abscissa represents the temperature of thesub-ambient treatment in C.

EXAMPLE II [Four test pieces of Pb-2 percent Sb were submitted to asolution treatment at 240 C. for one hour, and they were then rapidlycooled to 10 C. One of the test pieces was left at ambient temperature,and its hardness was checked as time transpired, it being observed, FIG.2, that it initially had a hardness of 6.0 HB and only at the end of 10days did it reach a value of 15.0 HB. Another test piece was introduced,immediately after cooling, into a bath at --10 C. and kept there for 3hours. The effect of this treatment was substantial, since the testpiece came to have a hardness of approximately 8.0 HB, and subsequentlyreached, at ambient temperature, the value of 15.0 HB in only 3 days.Another piece subjected to treatment for 1 day at --10 C. immediatelyafter tempering had, after this treatment, a hardness of 10.5 HB, and atthe end of 1 day at ambient temperature had a hardness of 17.0 HB, avalue never attained by the test pieces that did not undergo treatmentin cold. Finally, when the cold treatment lasts for two days at 10 C.,the values are spectacular, since at the end of this treatment the testpieces have a hardness of more than 13.0 HB, and with the passage oftime this value rises to nearly 20.0 HB.

Description of FIG. 2

In the figure there may be seen the evolution of hardness with time insome test pieces of an alloy of Pb-2 percent Sb which have been solutiontreated and cooled rapidly, and which immediately afterwards haveundergone the treatments indicated in FIG. 2.

5 What is claimed is: 1. A process for increasing the mechanicalresistance of elements, comprising the steps of:

providing a lead-base alloy comprising 0.3 %-,-11.5 by

weight of antimony; heating said alloy to a temperature sufiicient toobtain a solid solution of antimony in said lead-base alloy; coolingsaid alloy rapidly to a temperature of approxi mately -10 C. andmaintaining said alloy at said temperature for a period of from 3 hoursto 2 days and thereafter aging the alloy at ambient temperature toachieve the optimum desired hardness. 2. A process for increasing themechanical resistance of grids for accumulators comprising the steps of:

providing a lead-base alloy comprising from between 0.3% to 11.5% byweight of antimony;

melting said alloy;

casting said alloy into molds for forming said grids;

cooling said alloy rapidly to a temperature of approximately minus 10C.;

aging said alloy at said temperature for from 3 hours to 2 days andthereafter aging the alloy at ambient temperature to achieve the optimumdesired hardness.

References Cited UNITED STATES PATENTS 2,148,741 2/1939 Gonser 148-125 X3,008,853 11/1961 Borchers et al. 148-125 X 15 CHARLES N. LOVELL,Primary Examiner US. Cl. X.R. 148-125, 158

